1. Field of the Invention
The present invention relates to an anti-reflection film suitable for lowering reflection of light on a displaying surface of a display device, and relates to a display device provided with the anti-reflection film.
2. Description of Prior Art
As anti-reflection films for visible light, multi-layered films wherein plural transparent layers made of metal compounds (e.g., metal oxides) are superposed one on another have been employed. In the case that a monolayer film is employed instead of the multi-layered film, the monolayer film cannot effectively lower reflection of light having wide wavelength region (e.g., visible light), though the monolayer film can lower reflection of monochromatic light.
In the multi-layered film, increase of the number of the layers can enlarge a wavelength region of light to which the multi-layered film can apply. Therefore, the known anti-reflection film is the multi-layered film composed of two or more metal oxide layers which are deposited by means of deposition. However, formation of the multi-layered film requires a complicated process comprising a number of physical or chemical deposition procedures which correspond to the number of the metal compound layers having predetermined refractive index and thickness, under highly precise control of the thickness for each layer. Further, on the multi-layered film, a fluororesin layer is needed to be formed to improve stain resistance (e.g., resistance to formation of fingerprint onto the surface of the film).
Alternatively to the multi-layered film, it is known to use, as the anti-reflection film, a film in which refractive indices to air gradually vary in the thickness direction. Japanese Patent Provisional Publication No. 2(1990)-245702 describes an anti-reflection film comprising MgF2 micro particles and SiO2 micro particles having refractive index between those of glass and MgF2, in which blending ratio of the MgF2 micro particles and SiO2 micro particles varies in the thickness direction. In more detail, the anti-reflection film is prepared by coating a liquid containing a mixture of the MgF2 micro particles and SiO2 micro particles on a glass plate to reduce a ratio of the SiO2 micro particles to MgF2 micro particles. Thus formed anti-reflection film shows a little variation of refractive indices between its bottom and the upper surface of the glass plate. Therefore, the anti-reflection film shows a high anti-reflection effect.
Japanese Patent Provisional Publication No. 5(1993)-13021 also describes an anti-reflection film comprising two layers of MgF2 micro particles and SiO2 micro particles dispersed in ethyl silicate, a first layer containing MgF2 micro particles and SiO2 micro particles at ratio of 7/3 and a second layer containing MgF2 micro particles and SiO2 micro particles at ratio of 1/1. The first layer has a refractive index of 1.42 and the second layer has a refractive index of 1.44. Thus, the anti-reflection film does not show a satisfactory anti-reflection effect.
Japanese Patent Provisional Publication No. 7(1995)-92305 describes an anti-reflection film which comprises polymer particles (e.g., refractive index: 1.428) composed of core and shell. The film has a structure composed of an uneven surface portion of a low refractive index comprising air and the polymer particles and a portion comprising only the polymer particles. The core is composed of copolymer of methyl methacrylate, methacrylic acid, trifluoroethyl acrylate and N-isobutoxymethyl acrylate, and the shell is composed of copolymer of styrene, acrylic acid and butyl acrylate.
Japanese Patent Provisional Publication No. 7(1995)-168006 describes an anti-reflection film which has a structure composed of an uneven surface portion of a low refractive index comprising air and particles (e.g., MgF2), a portion (middle refractive index) comprising only the particles thereunder, and a portion comprising the particles and binder.
Japanese Patent Provisional Publications No. 2(1990)-245702, No. 5(1993)-13021, No. 7(1995)-92305 and No. 7(1995)-168006 mentioned above, all describe a film in which the refractive indices gradually vary in the thickness direction. However, the preparation of these films requires complicated procedures and skilled art, and further the films do not show a satisfactory anti-reflection effect.
It is an object of the invention to provide an anti-reflection film showing a high anti-reflection effect which can be prepared by a simple process.
It is further object of the invention is to provide an anti-reflection film having a low-refractive index layer which is improved in mechanical strength.
It is further object of the invention to provide an image display device provided with an anti-reflection film showing high anti-reflection effect which can be prepared by a simple process.
There is provided by the invention an anti-reflection film comprising a low refractive index layer which comprises a polymer binder and particles which are so deposited to superpose at least one micro particle on another micro particle, to form micro voids surrounded by the micro particles, said micro particles having a mean particle size of 5 to 200 nm.
Preferred embodiments of anti-reflection film of the invention are as follows:
1) The anti-reflection film wherein the particles are made of a fluorine-containing polymer (i.e., fluororesin), which preferably is a crystalline polymer.
2) The anti-reflection film wherein the particles are made of a fluorine-containing polymer, which contains a fluorine atom in an amount of not less than 0.30 weight fraction (preferably not less than 0.30 to 0.75 weight fraction, especially 0.35 to 0.75 weight fraction).
3) The anti-reflection film wherein the micro voids are contained in the range of 0.05 to 0.50 volume fraction (preferably 0.10 to 0.50 volume fraction) in the low refractive index layer.
4) The anti-reflection film wherein the particles are inorganic particles.
5) The anti-reflection film wherein the particles have a core-shell structure.
6) The anti-reflection film wherein the particles are treated with a silane-coupling agent.
7) The anti-reflection film wherein the fluorine-containing polymer is a homopolymer of perfluoro-2,2-dimethyl-1-dioxol or tetrafluoroethylene or a copolymer of these monomers.
8) The anti-reflection film which has a haze of 3 to 30%.
9) The anti-reflection film wherein the low refractive index layer is overlaid on a transparent film.
10) The anti-reflection film wherein the low refractive index layer has a thickness of 50 to 200 nm.
11) The anti-reflection film wherein each of the micro voids is closed with the binder polymer and the micro particles.
Further, there is provided by the invention an anti-reflection film comprising the low refractive index layer provided on a high refractive index layer having a refractive index higher than that of the low refractive index layer.
Furthermore, there is provided by the invention an anti-reflection film comprising the low refractive index layer provided on a high refractive index layer having a refractive index higher than that of the low refractive index layer, which is then provided on a middle refractive index layer having a refractive index between that of the low refractive index layer and that of the high refractive index layer.
The high refractive index layer provided with the low refractive index layer is preferably provided on a transparent film. The middle refractive index layer equipped with the low refractive index layer and the high refractive index layer is preferably provided on a transparent film.
Moreover, the invention provides an anti-reflection film comprising a low refractive index layer which has a refractive index of 1.20 to 1.40 and a thickness of 50 to 200 nm and is provided with micro voids of 0.02 to 0.28 volume fraction.
Furthermore, there is provided by the invention a display device provided with at least one anti-reflection film. The anti-reflection film is selected from the films described above.
The low refractive index layer of the anti-reflection film according to the invention is formed of a binder polymer and particles. Micro voids are formed by the particles. Each of the voids is preferably closed with the binder polymer and the particles. The low refractive index layer is a layer having refractive index lower than that of its material, and has an extremely low refractive index. Particularly, the anti-reflection film using the low refractive index layer together with at least one layers having high refractive index shows a high anti-reflection effect. The binder polymer has a function of adhering the particles to improve the mechanical strength of the low refractive index layer.